Using a specific anti-myosin antibody we shall examine myofibrillogenesis in cultured cardiac muscle cells under different conditions. This study is part of our ongoing efforts to understand the relationship between cytodifferentiation, myofibril assembly and cardiac morphogenesis. Additional studies will be done on cell surface coat turnover during heart development and the relationship between heart shape and an intact cardiac jelly. The cardiac jelly, which is the extracellular connective tissue of the tubular heart will be digested selectively by the injection of specific enzymes directly into the matrix.